I failed to find existing XmlBeans code that converts from JSON, despite mentions of JSON in relation to XmlBeans that are scattered all over the Internet.

No problem, all we need is a JSON parser that follows the SAX model, i.e., it generates an event when it parses the next JSON element. We will then generate the corresponding XML element in the XmlObject tree.

From the list of libraries on json.org, I have chosen Argo, which does exactly that with its JsonListener interface.